Weigh first, because guessing hides a slow loss
A senior guinea pig can lose a worrying share of her body weight while still looking round under a full coat. Weigh her weekly, on the same kitchen scales, at the same time of day, in the same small container, and write the figure down where you will actually compare it. A single low reading after a disturbed day means little; a steady drift down across two or three weeks is a finding, even if she still rushes for vegetables.
Set a personal threshold for action with your veterinarian — for many owners that is a clear continued fall, a sudden sharp drop, or weight loss paired with eating less hay — and treat crossing it as a booking trigger rather than a reason to watch for another month. Senior guinea pigs have little reserve, and the causes of weight loss are far easier to help early.
Feed for access and appetite, not just richness
Start with the staple she must never run out of: unlimited hay, piled fresh at floor level in more than one spot, because hay drives digestion and tooth wear and an arthritic pig may not walk to a single distant rack. Keep her familiar pellets in a heavy low dish and her vegetables in the places she already expects them, offered little and often through the day so each serving is fresh and tempting rather than one large portion that wilts.
It is tempting to answer weight loss with ever richer treats, but fatty extras can upset the gut and displace the hay she needs most, and they mask rather than fix a dental or medical cause. Better levers are access, freshness, and peace: feed her away from a pushier cage mate who may be finishing shared piles first, watch an actual meal to confirm she chews comfortably and at normal speed, and hand-offer a favourite vegetable at quiet times to learn whether appetite or opportunity is the problem.
Look for the reason the weight is moving
In a senior guinea pig, falling weight commonly points somewhere specific. Worn or uneven teeth make hay chewing slow and selective. Sore feet or stiff joints cut the number of trips to food and water. Long-term conditions affecting appetite, digestion, or the urinary system can all show up first as a thinner pig at the weekly weigh-in. Lump checks as you handle her, and notes on droppings, thirst, and breathing, turn a vague worry into useful evidence.
Book a guinea-pig-aware veterinarian promptly if the scale keeps falling, hay intake drops, droppings shrink, she drools or drops food, or she becomes quieter and hunches. Bring the weight log and feeding notes; a trend written down over weeks lets the vet judge severity and change far better than a recollection that she seems a bit thinner lately.
Make the routine easy for helpers to repeat
Senior weight care fails most often through inconsistency: different people feeding, nobody weighing, a weekend away resetting the pattern. Fix the weighing day, keep the log beside the scales, and make sure anyone feeding her knows which foods are her reliable favourites and which pile is weighed hay for the record versus extra bedding hay.
With KinsLe's Pet Passport, this guinea pig's weight log, vet findings and feeding routine sit together in one portable packet, so the next person to take her to an appointment can show the full trend rather than a single snapshot weight.